Skip Links

USGS - science for a changing world

Open–File Report 2009–1135

Magnetotelluric and Audiomagnetotelluric Groundwater Survey Along the Humu’ula Portion of Saddle Road Near and Around the Pohakuloa Training Area, Hawaii

By Herbert A. Pierce and Donald M. Thomas

Thumbnail of front cover and link to report (33,331 KB)


The Pohakuloa Training Area (PTA), operated by the U.S. Army on the Big Island of Hawaii, is in need of a reliable potable water supply to sustain ongoing operations by staff and trainees. In an effort to acquire baseline hydrologic data with which to develop a plan for providing that water, a series of magnetotelluric (MT) geophysical surveys was performed that spanned the Mauna Loa/Mauna Kea Saddle region of Hawaii Island. These surveys provided electrical resistivity profiles and resistivity maps at several elevations along the axis of the field measurements that can be interpreted to yield information on the depth to the water table. In 2004 a preliminary sequence of 23 audiomagnetotelluric (AMT) soundings was collected along Saddle Road extending from the Waikii Ranch area, west of the PTA, to Department of Hawaiian Home Lands Humu’ula properties east of the Mauna Kea access road. The results of those soundings showed that highly resistive rocks, consistent with dry basalts, were present to depths of at least one kilometer, the maximum depth to which the AMT technique can reliably reach in Hawaii’s rocks. A second survey was conducted in 2008 using MT instruments capable of recovering resistivity data to depths of several kilometers below sea level where saturated formations are known to exist. A total of 30 MT soundings was performed along a roughly east to west transect that extended from the (recently acquired) Keamuku PTA lands on the west to as far as the County of Hawaii’s upper Kaumana water supply well to the east. Inversion and processing of the field data yielded an electrical cross-section following the Saddle that roughly parallels the geologic contact between the Mauna Kea and Mauna Loa lavas. Several additional electrical sections were constructed normal to the main transect to investigate the three-dimensional nature of the contact. These resistivity data and models suggest that the elevation of saturated rock in places is 1,000 meters above mean sea level beneath the surveyed region. Highest elevations for water-saturated zones based upon preferred electrical models are located between training area 3 and training area 6 southwest of training area 4.

Revised October 1, 2009

Version 1.1

Posted August 2009

For additional information, contact:

Herbert A. Pierce
Eastern Earth Surface Processes Science Center
12201 Sunrise Valley Drive, Mail Stop 926A
Reston, VA  20192-0002

Part or all of this report is presented in Portable Document Format (PDF); the latest version of Adobe Reader or similar software is required to view it. Download the latest version of Adobe Reader, free of charge.

Suggested citation:

Pierce, H.A., and Thomas, D.M., 2009, Magnetotelluric and audiomagnetotelluric groundwater survey along the Humu’ula portion of Saddle Road near and around the Pohakuloa Training Area, Hawaii: U.S. Geological Survey Open-File Report 2009–1135, 160 p., on one CD. (Also available at

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo logo U.S. Department of the Interior | U.S. Geological Survey
[an error occurred while processing this directive] URL:
Page Contact Information: Contact USGS
Page Last Modified: Wednesday, December 07 2016, 09:59:27 PM